brittain



Jan. 31, 1956 F. H. BRITTAIN 2,732,903

LOUDSPEAKER DIAPHRAGM HAVING ASYMMETRIC INDENTATIONS Filed Nov. 29, 1952 2 Sheets-Sheet 2 FIG. 3.

INVENTGR EMA/O5 L/UfI-l FFITTHIN United States Patent LOUDSPEAKER DIAPHRAGM HAVING ASYMMETRIC INDENTATIONS Francis Hugh Brittain, Pinner, England, assignor to The General Electric Company Limited, London, England, a British company Application November 29, 1952, Serial No. 323,322

Claims priority, application Great Britain December 5, 1951 Claims. (Cl. 181-32) This invention relates to loudspeakers having a conical diaphragm. It is to be understood that in this specification the term conical includes frustoconical and the term cone includes a frustum of a cone.

Such loudspeakers in general exhibit a main resonance at some frequency within the upper part of the normal frequency range of the loudspeaker. It has been found that this resonance is due to one particular mode of vibration of the diaphragm which is controlled solely by the characteristics of the diaphragm and is not affected by other parts of the loudspeaker. This may be explained in the following manner. Consider a narrow sector of the diaphragm extending from its apex to its base. This sector may be considered as being held on pivots at both ends, and if the inner end is vibrated at an appropriate frequency the sector can be made to resonate, the resonant frequency depending upon the length, mass and stiffness of the sector. In the fundamental mode there is one antinode which will not be at the centre of the length of the sector but will be nearer the outer end of the sector owing to the fact that the curvature, and therefore the stiffness, of the sector varies along its length. A conventional diaphragm in the form of a right circular cone may be considered as being composed of a number of such sectors, all identical and having the same resonant frequency in the fundamental mode, and the loudspeaker will therefore exhibit a promincnt peak in its frequency response curve at this frequency.

It is an object of the present invention to provide a loudspeaker having a conical diaphragm in which the peak in the frequency response curve referred to above is considerably reduced in prominence by utilising simple deformation of the diaphragm, capable of being carried out by simple production techniques and resulting in a minimum disturbance of the other characteristics of the diaphragm.

According to the invention, a loudspeaker has a conical diaphragm of a single material which is indented at three (and only three) places in the outer half of the diaphragm, two of the indentations being disposed close to each other and the third being disposed diametrically opposite the gap between the other two.

One arrangement in accordance with the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figures 1(a) and 1(1)) are explanatory diagrams;

Figure 2 is a sectional View of part of a 6" moving coil loudspeaker having a conical metal diaphragm; and

Figure 3 is an end elevation of the diaphragm of the loudspeaker shown in Figure 2, the view being in the direction indicated by the arrow A in Figure 2.

it has been found that a 6" moving coil loudspeaker having a metal diaphragm in the form of a right circular cone and of conventional construction has a frequency response curve similar to that shown in Figure 1(a) of the accompanying drawings in which frequency in cycles per second is plotted as abscissa and the relative response of the loudspeaker in decibels is plotted as ordinate. As will be seen from Figure 1(a), a very prominent peak in the frequency response curve occurs at a frequency of about 8.5 kilocycles per second. In order to eliminate this peak in the frequency response curve, the construction shown in Figures 2 and 3 of the drawings is adapted for the diaphragm of the loudspeaker.

Referring to Figures 2 and 3, the diaphragm 1 of the loudspeaker is generally in the form of a right circular metal cone having an axial length of 1%" and a conical angle of the diaphragm 1 being welded to a former 2 carrying the voice coil 3, which is mounted in a gap in the magnetic circuit 4. In accordance with the invention the diaphragm 1 is distorted in the following manner from its conical form at three regions in the outer half of the diaphragm 1. At each region an incision is made through the diaphragm 1 extending along a circle which is spaced approximately /2 along the slant height of the diaphragm 1 from the outer periphery of the diaphragm 1; two of the incisions have a length of 1 7 and are spaced apart along the circle, while the third incision is disposed diametrically opposite the gap between the first two incisions and has a length of A3. The free edges formed on the inner sides of the incisions are pushed out into the interior of the diaphragm 1 to form louvres 5, 6 and 7, the louvre 5 formed in the region of the short incision having a length of along the slant height of the diaphragm 1, and the louvres 6 and 7 formed in the regions of the long incisions having lengths of 1 along the slant height of the diaphragm 1. This distortion has the effect of increasing the stiffness of the diaphragm 1 in the regions of the louvres 5, 6 and 7 as compared with the rest of the diaphragm 1, and thus the diaphragm 1 is composed of a number of sectors some of which have resonant frequencies in the fundamental mode which differ appreciably from the resonant frequencies in the fundamental mode of others of the sectors. It will be appreciated that the louvres 5, 6 and 7 are disposed asymmetrically around the axis of the diaphragm 1, although the diaphragm 1 is bilaterally symmetrical about the diametric plane represented by the line B in Figure 3, that is the plane corresponding to the section of Figure 2. In order to prevent excessive high frequency radiation from the louvres 5, 6 and 7, which would cause a secondary peak in the frequency response curve of the loudspeaker, it is desirable that the louvres 5, 6 and 7 should be made comparativelyrigid. This may be achieved by ensuring that the freeedges of the louvres 5, 6 and 7 are not in the form of simple arches, but are in the form of curves whose cur-- vatures are in opposite directions at different points along;

their lengths.

It will be noted that, in addition to the peak referred to above, the frequency response curve shown in Figure 1(a) exhibits a dip at a frequency of about 2.5 kilocycles per second. In order to eliminate this dip in the frequency response curve, the loudspeaker is provided with a rigid plug 8 in accordance with co-pending patent application No. 312,111, filed September 29, 1952, now Patent No. 2,706,529, issued April 19, 1955, the plug 8 being secured by means of a screw 9 to the part 10 of the magnetic circuit disposed inside the former 2 so as to be disposed within the space bounded by the diaphragm 1. The plug 8 is in the shape of two coaxial circular cylinders 11 and 12 joined by a right circular conical part 13. As specified in said Patent No. 2,706,529, the plug 8 should extend from the vicinity of the apex of the diaphragm 1 so as to occupy between 10% and 20% of the space bounded by the diaphragm 1, at least that portion of the plug 8 which is nearest I .3 the apex of the diaphragm '1 having a surface disposed close "to the diaphragm 1. The plug 8 may be of any convenient material, for example wood, metal or one of the materials commonly known as plastics.

With the construction described above and shown in Figure 2, the loudspeaker has a frequency response curve similar to that shown in "Figure 1(5), and it can be seen that both the peak and the dip in .the frequency response curve have been substantially eliminated, so that the frequency response curve is substantially flat within the range of frequencies extending from 50 cycles per second to 10 kilocycles per second.

It will be appreciated that a loudspeaker having a conical diaphragm of some non-metallic material such as paper may be treated in a similar manner .to that described above. In this case .the resonance in the mode described above is not only one which occurs within the upper part of the normal frequency range of the loudspeaker; however this resonance produces the main peak in the high frequency portions of the frequency response curve of the loudspeaker, and the use of the invention enables a considerable flattening of this portion of the frequency response curve to be effected.

I claim:

1. A loudspeaker having a conical diaphragm of a single material which is indented at only three places in the outer half of the diaphragm, two of the indentations being disposed close to each other and the third being disposed diametrically opposite the gap between the other-two.

2. A loudspeaker as set forth in claim 1, in which said third indentation is smaller than the other two.

3. A loudspeaker as set forth in claim 2, in which said indentations are in the form of louvres.

4. A loudspeaker as set forth in claim .1, in which the diaphragm is of metal.

5. A loudspeaker having a conical diaphragm of a single material which is indented at only three places in the outer half of the diaphragm, two of the indentations being disposed close to .each other and the third being disposed diametrically opposite the gap between the other two, and the indentations being in the form of louvres the free edges of which are in the form of curves whose curvatures are in opposite directions at different points along their lengths.

References Cited in the file of this patent UNITED STATES PATENTS 1,298,290 Chalmers Mar. 25, 1919 1,743,767 Focht Ian. 14, 1930 1,916,223 Kalin July 4, 1933 1,917,309 Leopold July 11, 1933 2,022,060 Swickard Nov. 26, 1935 2,124,597 Tiedje July 26, 1938 2,657,758 Varnet Nov. 3, 1953 mi. m l m 

